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Re: container_of macro...

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Andrey Tarasevich <andreytarasevich@hotmail.com> writes:

> On 11/2/2022 7:15 AM, tylo wrote:
>
>> tirsdag 1. november 2022 kl.  12:06:17 UTC+1 skrev Ben Bacarisse:
>>
>>>> (type*)0->member dereferences a null pointer.  As a result, such
>>>> code has undefined behavior.
>>>
>>> It's in the unevaluated operand of the sizeof operator.  The
>>> comparison is there just to provoke type-checking (the main thrust
>>> of this subthread) and I imagine the sizeof is there to avoid the
>>> implied UB.
>>>
>>> -- >> Ben.
>>
>> Yes, sizeof is also there to avoid ptr being evaluated twice.
>
> No.  It is there to inject a dummy comparison into an expression.
>
>> Although not relevant here as sizeof() doesn't evaluate its
>> argument, the expression &((Type*)0)->member evaluates to a
>> compile time constant even by non-optimizing compilers, as they
>> always do basic constant folding optimizations.
>
> No, no, and no.
>
> Firstly, what is a constant expression and what isn't is defined by
> the language spec.  [...]
>
> Secondly, the above expression does not evaluate to a "compile time
> constant".  [...]

This response isn't exactly right.  Both gcc and clang accept an
expression of the form &((Type*)0)->member as a constant expression.
In particular, a source file

    typedef struct { int member; } Type;

    const int *x = & ((Type*)0)->member;

is accepted, in a conforming mode, including -pedantic-errors, and
without causing any diagnostics (and indeed the value given to x is
a compile-time constant).  Such expressions don't /have/ to be
constant expressions but they /can/ be constant expressions - the C
standard explicitly allows additional forms of constant expressions,
beyond those described in the standard, to be accepted by a
conforming implementation.  Similarly, the value of the expression
doesn't /have/ to have any particular value but it /can/ be given
the value most people would expect.  Certainly it is true that the
previously quoted expression /might not/ evaluate to a compile time
constant, but saying it /does not/ evaluate to a compile time
constant is wrong:  sometimes it does, even if under different
circumstances it would not.

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Thread

Re: container_of macro... tylo <tylovset@gmail.com> - 2022-10-31 03:27 -0700
  Re: container_of macro... James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-11-01 00:20 -0400
    Re: container_of macro... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-11-01 11:06 +0000
      Re: container_of macro... tylo <tylovset@gmail.com> - 2022-11-02 07:15 -0700
        Re: container_of macro... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-11-02 16:00 +0000
          Re: container_of macro... Richard Damon <Richard@Damon-Family.org> - 2022-11-02 21:00 -0400
            Re: container_of macro... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-11-03 01:03 +0000
              Re: container_of macro... Richard Damon <Richard@Damon-Family.org> - 2022-11-02 21:43 -0400
                Re: container_of macro... Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-11-03 15:29 +0000
                Re: container_of macro... tylo <tylovset@gmail.com> - 2022-11-16 03:38 -0800
            Re: container_of macro... James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-11-03 00:15 -0400
          Re: container_of macro... Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-11-19 06:35 -0800
        Re: container_of macro... David Brown <david.brown@hesbynett.no> - 2022-11-02 17:31 +0100
        Re: container_of macro... Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-11-02 10:09 -0700
          Re: container_of macro... Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-11-19 11:27 -0800
      Re: container_of macro... James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-11-03 00:14 -0400
        Re: container_of macro... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-11-02 21:28 -0700
          Re: container_of macro... "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-11-02 21:29 -0700

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